I forget where I found this long ago, but here is an older standard (drafted 1975, reaffirmed 1993) that has both component symbols and reference designators. See PDF page 231 (document page 211) for the start of the section on reference designators (called Class Designation Letters in the document). See also PDF page 261 cross-referencing this document (Y32.2) to IEC 113-2. (Even the standards don't agree, as usual.)
IEEE Std 315-1975 & ANSI Y32.2-1975 - electric-symbols.pdf (3.2 MB)
It is very detailed (for example showing what symbols to use with switches to indicate what activates them; thermal, temperature, push, pull, etc) but possibly outdated. But even being outdated if you read between the lines you can some historical context for an understanding of why things are they way they are today.
For example an IC having U as a designator prefix is because it is considered an assembly (some one built it), but it isn't something that can then be disassembled. Thus "inseparable assembly". One could argue that vacuum tubes are similar (because they are), but my theory has two parts: (remember this is IMHO, and I'm not a historian...)
- First, ICs were late comers to the scene, so instead of making a new designator for something that had yet to prove itself in the industry, it was just shuffled into an existing category that it kind of fits into.
- Second, the name "integrated circuit" alludes to a separate entity. It's a full circuit unto itself so could be considered a separate assembly, thus either A or U, but again because it isn't able to be disassembled that would make it U.